1. Field of the Invention
This invention relates to solid-state lighting applications which comprise light emitting diodes (LEDs) which include a light emitting phosphor, photoluminescent material, to generate light of a desired color, that is in a different part of the wavelength spectrum from the LEDs. In particular, although not exclusively, the invention concerns LED-based lighting arrangements which generate light in the visible part of the spectrum and in particular, although not exclusively white light. Moreover the invention provides an optical component for such a lighting arrangement and methods of fabricating a lighting arrangement and an optical component. Furthermore the invention provides a phosphor material for coating an optical component or as a part of optical designs in lighting arrangements.
2. State of the Art
In the context of this patent application light is defined as electromagnetic radiation in a wavelength range 300 nm (Ultraviolet) to 1000 nm (Infrared). Primarily, although not exclusively the invention concerns lighting arrangements which emit light in the visible part of the spectrum that is 380 to 750 nm.
White light emitting diodes (LEDs) are known in the art and are a relatively recent innovation. It was not until LEDs emitting in the blue/ultraviolet of the electromagnetic spectrum were developed that it became practical to develop white light sources based on LEDs. As is known white light generating LEDs (“white LEDs”) include a phosphor, that is a photoluminescent material, which absorbs a portion of the radiation emitted by the LED and re-emits radiation of a different color (wavelength). For example the LED emits blue light in the visible part of the spectrum and the phosphor re-emits yellow light. Alternatively the phosphor can emit a combination of green and red light, green and yellow or yellow and red light. The portion of the visible blue light emitted by the LED which is not absorbed by the phosphor mixes with the yellow light emitted to provide light which appears to the eye as being white. A known yellow phosphor is a YAG-based phosphor having a main emission wavelength peak that varies in wavelength range from 530 to 590 nm depending on the composition of the phosphors. Further examples of phosphors are described in our co-pending patent application US 2006/0028122 in which the photoluminescent materials have a formula A2SiO4:Eu2+D where A is a divalent metal selected from the group consisting of Sr, Ca, Ba, Mg, Zn and Cd and D is a dopant selected from the group consisting of F, Cl, Br, I, P, S and N. Such phosphors emit light of intensities that are greater than either known YAG compounds or silicate-based phosphors.
It is predicted that white LEDs could potentially replace incandescent light sources due to their long operating lifetimes, typically many 100,000 of hours, and their high efficiency. Already high brightness LEDs are used in vehicle brake lights and indicators as well as traffic lights and flash lights.
To increase the intensity of light emitted from an LED it is known to include a lens made of a plastics material or glass to focus the light emission and to thereby increase intensity. Referring to FIG. 1 a high brightness white LED 2 is shown. The LED 2 comprises an LED chip 4 which is mounted within a plastic or metal reflection cup 6 and the LED chip is then encapsulated within an encapsulating material, typically an epoxy resin 8. The encapsulation material includes the phosphor material for providing color conversion. Typically the inner surface of the cup 6 is silvered to reflect stray light towards a lens 10 which is mounted on the surface of the encapsulating epoxy resin 8.
The inventor has appreciated that such an arrangement has limitations and the present invention arose in an endeavor to mitigate, at least in part, these limitations. For example for high intensity LEDs having a high intensity output larger than 1 W, the high temperature at the output of the LED combined with its close proximity the phosphor material can give rise to a light characteristic which is temperature dependent and in some cases thermal degradation of the phosphor material can occur. Moreover the uniformity of color of light emitted by such LEDs can be difficult to maintain with the phosphor distributed within the epoxy resin since light passing through different path lengths will encounter and be absorbed by differing amounts of phosphor. Furthermore the fabrication of such LEDs is time consuming due to the encapsulation and subsequent placement of the lens.